Since this is a list of a collectible, there might be need for some clarifications in my explanations, so please don't hesitate to tell me your ideas!
This achievement is granted to those who unlock all the doors with a knife/shiv. A shiv is always good to have, especially with these doors scattered around, so always have a couple of these on you.
13 doors to unlock in total.
Because of New Game + mode, numbers are carried over from your previous playthroughs, so don't fret if you've missed a door or two.
In main menu - bonuses - statistics a number is shown for doors unlocked, but it doesn't show which ones you might have missed.
I'll point out the locations of these doors below in the spoilertag, as there's bound to be spoilers in the description of place and people. It's advised to only open this spoilertag if you've already played through the game once.
*** Spoiler - click to reveal ***
[]Shiv Door 1
Location: Outskirts – Downtown (Abandoned Building)
After you've gotten past the group of Clickers, this door is located at fifth floor. From the hallway , go through the door located on the left wall, you'll see the door at the end wall.
[]Shiv Door 2
Location: Outskirts – Museum (Second Floor)
Once again, after you've dealt with a group of Clickers. This door is located at the top of the museum.
[]Shiv Door 3
Location: Bill’s Town – The Woods (First Floor)
This door is located in a small alley to your left. A pinpoint to location is this is near the building where you have to pull the plank and place it to cross between buildings. After taking down the two Clickers, look for the door. There's a lot of supplies, parts and ammo behind this door.
[]Shiv Door 4
Location: Bill’s Town – Main Street
When you've gotten past Bill's snap trap, the door is in one of the buildings. When Ellie starts talking about the traps, you're getting close, the door is somewhere to your left, assuming you're heading in the right direction.
[]Shiv Door 5
Location: Alone & Forsaken – Before the Back Alley
After escaping the Hunters in the warehouse building, you'll find the door in a back alley. Open it before proceeding to the next area.
[]Shiv Door 6
Location: Alone & Forsaken – Before the Hotel
Before you take on the swamp to reach the hotel, the door is on your left. Behind it is supplements, gear to upgrade your weapons and ammo.
[]Shiv Door 7
Location: Financial District – Streets
Notice the vehicles blazing bullets at you? This door is located in one of the restaurants here. Stay down to cover from the bullets.
[]Shiv Door 8
Location: Escape in the City – Apartments
After you've met up with Henry and Sam, this door is in one of the rooms in the apartment. There's a training manual behind it.
[]Shiv Door 9
Location: Tommy’s Dam – Hydroelectric Dam
This door is not easy to miss as it's near the water bridge where you have to activate the lever to raise the platformin order to get across.
[]Shiv Door 10
Location: The University – Science Building (Second Floor)
Second floor of the science building besides some empty shelves. Contains manual, ammo and supplements.
[]Shiv Door 11
Location: Lakeside Resort – Cabin
There's a building across the road from the gas station. Get inside, climb through the broken bathroom window - the door should be to your left.
[]Shiv Door 12
Location:The Bus Depot – Underground Tunnel
After getting past the nightmare of Clickers, Infected and Bloaters, you'll find this door in one of the rooms after helping Ellie opening a door. Don't follow her, you'll find the door to your right.
[]Shiv Door 13
Location: The Firefly Lab – Hospital
Reacing the 6th floor, scourge the floor a door behind a counter with a window pane. That should be the last door.